As is generally well known in the railway industry art, during hot weather, particularly in certain areas of the country, railroad rails will expand and depending upon the degree of such expansion they can buckle. Such buckling can cause a number of difficulties not the least of which is derailment of a train running of such rails. These derailments are not only costly to the railroads due to equipment damage but also such derailments can cause a number of serious injuries or even death, as for example, when such derailment involves a passenger train.
As has been reported, track buckling is formation of large lateral misalignments in continuous welded rail (CWR) track, often resulting in catastrophic these derailments. Both curved and tangent tracks are susceptible to buckling with typical curve buckle amplitudes ranging from 6″-14″ and tangent buckles from 12″-28″. Buckles are typically caused by a combination of three major factors: high compressive forces, weakened track conditions, and vehicle loads (train dynamics).
Compressive forces result from stresses induced in a constrained rail by temperature above its “stress free” state, and from mechanical sources such as braking, rolling friction and wheel flanging on curves. The temperature of the rail at the “stress-free” state is known as the rail neutral temperature (i.e. the temperature at which the rail experiences zero longitudinal force). Initially, the rail's installation temperature or “anchoring temperature” is the rail's neutral temperature. Hence, at rail temperatures above the neutral, compressive forces are generated, and at temperatures below the neutral, tensile forces are developed. Track maintenance practices address the high thermal load problem by anchoring the rail at (neutral) temperature of 95-110° F. This high neutral temperature range prevents the generation of excessively high buckling forces even when the rail temperatures reach 130-150° F.
Another significant problem that can be attributed to such rail expansion is that it will oftentimes cause the railroad ties supporting the rails to shift and thereby loosen the ballast around the ties. This situation occurs because the rails and tie plates which support such rails are secured to the ties in a relatively tight fashion. The shifting of ties can also contribute to derailments and will at least add significantly to the cost of maintaining the track bed in a serviceable condition.
Weakened track conditions impacting the tracks buckling potential include: reduced track Resistance, lateral alignment defects, and lowered rail neutral temperature. Track resistance is the ability of the ballast, ties and fasteners to provide lateral and longitudinal strength to maintain track stability. Resistance is lowered if ballast is missing from under the ties, in the crib or from the shoulder. A full ballast section is important, especially on curves. Adequate ballast in the high side in curves should be on the order of 12″-18″ to provide adequate lateral strength. Ballast on the low side is important because inward (pulling-in) movement in cold weather could lead to line defects and lowering of neutral temperature which could lead to a buckle when higher temperature rises occur in early spring. Track resistance is also lowered when ballast is disturbed. Surfacing, tie renewal and undercutting operations will weaken ballast resistance by as much as 40%-60% of undisturbed track. It is a usual industry practice to restrict train speed to minimize train forces while ballast strength is being restored either by traffic or by mechanical consolidation means. Longitudinal resistance offered to the rail/tie structure by adequate rail anchoring is important to prevent rail running and hence the decrease of rail neutral temperature.
Lateral alignment defects also reduce the track's buckling strength because buckles tend to initiate at alignment deviations. The larger the line defect, the more buckling prone the track will be. Alignment errors must be corrected in hot weather and in early spring when curves tend to realign themselves from a winter “pull-in” condition. Buckles can also initiate at bad, crooked welds.
Maintaining a stable and high rail neutral temperature is critical for buckling prevention. Neutral or force-free temperature of CWR is usually different from initial installation or anchoring temperature. This difference is attributed to several factors, including rail longitudinal movement, track lateral shift/radial breathing in curves, track vertical settlement, and maintenance activities. Rail longitudinal movement (creep) is due to train braking and traction forces, or to differential thermal forces (sun and shade). Track lateral shift can be caused by excessive truck hunting, and by lateral forces generated by curving or by lateral misalignments. Compressive and tensile forces can cause radial breathing of curves especially in weak ballast conditions. Vertical differential settlement of rails can occur on new or recently surfaced track, or in areas of weak subgrade conditions. Maintenance operations influencing neutral temperature changes include: lifting, lining, and tamping, replacing broken rail, de-stressing, and installing CWR in cold weather. Research to date has shown that typical CWR rail installation (stress-free) temperatures of 100° F. can reduce in service to 50-60° F. due to these effects.
Track buckles usually initiate at small alignment deviations. Wheel loads and train action (dynamic uplift wave) tend to increase its size to levels which trigger the buckling process. Most buckling derailments tend occur deep in a train. Vehicles contribute to buckling by exerting lateral wheel forces in a curve. Lateral forces can also occur in tangent track from car movement caused by line or surface deviations or track hunting. The track must absorb this energy. Slack action, heavy dynamic braking and emergency brake applications can trigger a buckle. It is important to inspect track after a train passage in hot weather, especially if the track has recently been disturbed.
The above is a brief summary of the track buckling problem in terms of the three major causal factors: high compressive forces, weakened track conditions, and vehicle loads (train dynamics).
Over one five year period of time, from 1997 to 2002, statistics indicate an average of 38 derailments a year with an increasing yearly damage level to as high as $17 million in 2002. Currently there are no FRA safety performance standards in place addressing CWR buckling safety.
It has been further reported that as the temperature rises above preselected temperatures the train speed must be lowered and in some cases trains can only run at night.
In one prior art type effort to alleviate this situation, which is known to applicant, the rails were painted white in order to decrease the amount of heat being absorbed by such rails. Although this did improve somewhat the resistance of the painted rails to heating up it was not entirely successful, however, because such paint became covered with grease and grime due to nature of the outdoor environment such rails are exposed too.